1. Field of the Invention
The present invention relates to a hydrodynamic bearing, a spindle motor using the same and a disc drive apparatus provided with the spindle motor.
2. Description of the Prior Art
As a bearing for rotatably supporting a rotor of a spindle motor used for a disc drive apparatus such as a hard disc drive or the like, it is started to employ a hydrodynamic bearing which is quiet, has a low vibration and can obtain a stable rotation.
As the hydrodynamic bearing mentioned above, for example, there is U.S. Pat. No. 5,423,612. FIG. 4 shows a partly schematic structure of U.S. Pat. No. 5,423,612 mentioned above as a partly enlarged cross sectional view.
A thrust plate 102 is fixed to a fixed shaft 100, and a lubricating oil 106 corresponding to a working fluid is retained between an outer peripheral surface of the fixed shaft 100 positioned in a lower side of the thrust plate 102 in an axial direction, and an outer peripheral surface of a rotor 104 opposing to the outer peripheral surface in a radial direction. Further, a radial bearing portion 108 is structured by forming a radial dynamic pressure generating groove 104a which generates a supporting pressure for supporting a radial load applied to the rotor 104 by moving the lubricating oil 106 in a predetermined direction, at a rotating time of the rotor 104, on an inner peripheral surface of the rotor 104.
Further, the lubricating oil 106 is retained continuously between an axial lower end surface of the thrust plate 102 and an axial upper end surface of the rotor 104 opposing to the axial lower end surface in the axial direction, and between an axial upper end surface of the thrust plate 102 and a thrust bush 110 mounted to the rotor 104 opposing to the axial upper end surface in the axial direction. Further, a pair of thrust bearing portions 112 and 114 are structured by forming thrust dynamic pressure generating grooves 102a and 102b which generate a supporting pressure for supporting an axial load applied to the rotor 104 by moving the lubricating oil 106 in a predetermined direction, at a rotating time of the rotor 104, on the lower end surface and the upper end surface of the thrust plate 102.
In the case of the hydrodynamic bearing having the lubricating oil 106 as the working fluid, a taper-shaped seal gap is formed in an end portion of the bearing portion, and a gas-liquid interface between the lubricating oil and the air is formed and retained within the taper-shaped seal gap (refer to a capillary seal portion 116 in FIG. 4).
In the capillary seal portion in the hydrodynamic bearing, a difference is generated in a capillary force of the lubricating oil in correspondence to a position where the boundary surface of the lubricating oil is formed, by being progressively expanding a gap dimension of the gap formed within the capillary seal portion in correspondence to being apart from the bearing portion. Accordingly, in the case that an amount of the lubricating oil retained in the bearing portion is reduced, lubricating oil is supplied from the capillary seal portion, and in the case that a volume of the lubricating oil retained within the bearing portion is increased due to a temperature increase of the like, the increased amount is received.
Further, in a capillary seal portion 116 illustrated in FIG. 4, a gas-liquid interface 118 between the air and the lubricating oil 106 which is communicated with an external portion of the bearing portion through the radial gap between the shaft 100 and the thrust bush 110 is formed in a meniscus shape directed to an inner side in a radial direction. In accordance with the structure, since a centrifugal force is applied so as to press the gas-liquid interface 118 to an outer side in the radial direction, that is, toward the thrust bearing portions 112 and 114, at a time when the rotor 104 rotates at a high speed, it is possible to more effectively prevent the lubricating oil 106 from leaking to the external portion of the bearing portion.
In this case, during the use of the spindle motor mentioned above, a vibration and an impact force (hereinafter, described as an external shock) is applied to the rotor 104 from the external portion due to various causes. In the case that the external shock is applied at the rotating time of the rotor 104, there is a low possibility that a vibration and a turbulence in attitude are generated in the rotor 104 on the basis of a damping effect caused by a dynamic pressure generated in the thrust bearings 102a and 102b and the radial bearing portion 108 and a viscosity of the lubricating oil 106. However, since no dynamic pressure is generated at a time when the rotation of the spindle motor is stopped, only the viscosity of the lubricating oil 106 stands against the external shock of the motor.
In the case of the structure shown in FIG. 4, when the external shock is applied to the spindle motor in a state in which the spindle motor stops, the rotor 104 oscillates vertically in the axial direction as shown by an arrow A. In this case, since the gap in the axial direction is formed in each of the thrust bearing portions 102a and 102b and the capillary seal portion 116, the axial dimension of the gap is changed in both of the thrust bearing portions 102a and 102b and the capillary seal portion 116, on the basis of the oscillation of the rotor 104 caused by the external shock. In addition, the gas-liquid interface 118 oscillates in a radial direction as shown by an arrow B, in accordance with a change of the gap in the axial dimension.
In the case that the rotor 104 oscillates downward in the axial direction as shown by the arrow A at this time, the axial dimension of the seal gap is narrowed in the capillary seal portion 116, in addition to the gap of the thrust bearing portion 114 formed between the thrust plate 102 and the thrust bush 110. Accordingly, the gas-liquid interface 118 of the capillary seal portion 116 is in a state of swelling out to the inverse direction to the meniscus shape illustrated in FIG. 4, that is, to the inner side in the radial direction. Further, in the case that the external shock applied to the rotor 104 is more than a surface tension of the lubricating oil 106, a breakage of the gas-liquid interface 118 is caused, and the lubricating oil 106 is scattered to the external portion of the bearing.
The lubricating oil 106 scattered to the external portion of the bearing portion by the breakage of the gas-liquid interface 118 contaminates the internal portion and the external portion of the spindle motor in accordance with the rotation of the rotor 104. For example, in the case that the scattered lubricating oil 106 is attached to a recording surface of a recording disc such as a hard disc or the like mounted to the rotor 104, and a head reading and writing a recording date with respect to the recording disc, it is hard to read and write the recording data so as to cause generation of a read and write error in the recording data, so that a reliability of the disc drive apparatus is deteriorated.
Further, when the lubricating oil 106 is scattered to the external portion of the bearing portion, a retaining amount of the lubricating oil 106 is insufficient in the internal portion of the bearing portion, a bearing rigidity is lowered, and a rotation support of the rotor 104 is unstable. In addition, a contact sliding is generated between the thrust plate 102, and the shaft 100 and the rotor 104 due to a deplete of the retaining amount in the lubricating oil 106, so that there is a case that a seizure is generated.
In this case, in U.S. Pat. No. 2003-0030222, there is disclosed a structure in which the capillary seal portion is formed not in a direction orthogonal to the rotation axis but in a direction inclined to the rotation axis. In accordance with the structure of the capillary seal portion mentioned above, it is possible to make a volumetric capacity of the capillary seal portion larger than that of the structure in FIG. 4, however, since the capillary seal portion is affected by the change of the gap dimension caused by the oscillation of the rotor in the axial direction, a resistance property comes short against the external shock at a time when the rotation of the motor stops as mentioned above.